I would like to set the PDF Export Preset ahead of time, and be able to modify a couple aspects of that preset. [CS3]

For example, I could set the preset before export to "Smallest File Size" then also turn on the "export spreads" option.

If I try to set the Spreads option before exporting, and also say "using "SFS" in the export line, the "SFS" preset overrides/cancels out the Spread setting I previously set, it seems. Which is weird because I CAN set the security options before exporting a document and the security will override the preset I use after that command.

But I'm not seeing an obvious way to "set active PDF Preset to something". It only works in the "export format PDF type to X using SFS without showing options".

If I understand what you want to do correctly, you'll need to get the properties of the PDF export preset, set the properties of PDF export preferences to them, modify any you want, then export without using the preset.